The development of vaccines against highly variable pathogens such as HCV and HIV remains a major challenge due to the ability of these pathogens to rapidly mutate to evade critical immune responses. Unfortunately, efforts to develop these vaccines have met with little success, in part due to the inability to identify the precise correlates of effective immunity. CD8+ T cells (CTL) appear critical to the control of many highly variable pathogens including HCV, which is supported by the association of particular HLA class I alleles with control of HIV, malaria, and HCV. Recent data in HIV and HCV now begin to suggest that this control is associated with the ability of particular CD8+ T cell responses to target regions in which viral escape is impaired due to the impact of escape mutations on viral replication. The goal of this proposal is to utilize viral sequence evolution to identify those CD8+ T cell responses capable of exerting the greatest selective pressure upon HCV as a surrogate marker of CD8+ T cell efficacy. We plan to comprehensively examine the frequency and location of CTL escape mutations across HCV to identify CDS responses (defined and novel) exerting selective pressure, determine the extrinsic host and viral factors that influence CTL escape, and measure the impact of mutations on viral replication. Applying comprehensive viral sequencing and functional immunology to these questions will help elucidate the impact of particular CDS responses and escape mutations on control of HCV and provide unique insight into the specificity and function of CDS responses most critical to the control of HCV. Therefore, these efforts will help identify immune responses critical to the control of HCV, as a model for other highly variable pathogens, in order to guide the design of effective vaccines against these difficult to control pathogens.